[1] The name Glasites or Glassites was generally used in Scotland; in
England and America the name Sandemanians was more common.
GLASS (O. E. _glaes_, cf. Ger. _Glas_, perhaps derived from an old
Teutonic root _gla-_, a variant of _glo-_, having the general sense of
shining, cf. "glare," "glow"), a hard substance, usually transparent or
translucent, which from a fluid condition at a high temperature has
passed to a solid condition with sufficient rapidity to prevent the
formation of visible crystals. There are many varieties of glass
differing widely in chemical composition and in physical qualities. Most
varieties, however, have certain qualities in common. They pass through
a viscous stage in cooling from a state of fluidity; they develop
effects of colour when the glass mixtures are fused with certain
metallic oxides; they are, when cold, bad conductors both of electricity
and heat, they are easily fractured by a blow or shock and show a
conchoidal fracture; they are but slightly affected by ordinary
solvents, but are readily attacked by hydrofluoric acid.
The structure of glass has been the subject of repeated investigations.
The theory most widely accepted at present is that glass is a quickly
solidified solution, in which silica, silicates, borates, phosphates and
aluminates may be either solvents or solutes, and metallic oxides and
metals may be held either in solution or in suspension. Long experience
has fixed the mixtures, so far as ordinary furnace temperatures are
concerned, which produce the varieties of glass in common use. The
essential materials of which these mixtures are made are, for English
flint glass, sand, carbonate of potash and red lead; for plate and sheet
glass, sand, carbonate or sulphate of soda and carbonate of lime; and
for Bohemian glass, sand, carbonate of potash and carbonate of lime. It
is convenient to treat these glasses as "normal" glasses, but they are
in reality mixtures of silicates, and cannot rightly be regarded as
definite chemical compounds or represented by definite chemical
formulae.
The knowledge of the chemistry of glass-making has been considerably
widened by Dr F. O. Schott's experiments at the Jena glass-works. The
commercial success of these works has demonstrated the value of pure
science to manufactures.
The recent large increase in the number of varieties of glass has been
chiefly due to developments in the manufacture of optical glass. Glasses
possessing special qualities have been required, and have been supplied
by the introduction of new combinations of materials. The range of the
specific gravity of glasses from 2.5 to 5.0 illustrates the effect of
modified compositions. In the same way glass can be rendered more or
less fusible, and its stability can be increased both in relation to
extremes of temperature and to the chemical action of solvents.
The fluidity of glass at a high temperature renders possible the
processes of ladelling, pouring, casting and stirring. A mass of glass
in a viscous state can be rolled with an iron roller like dough; can be
rendered hollow by the pressure of the human breath or by compressed
air; can be forced by air pressure, or by a mechanically driven plunger,
to take the shape and impression of a mould; and can be almost
indefinitely extended as solid rod or as hollow tube. So extensible is
viscous glass that it can be drawn out into a filament sufficiently fine
and elastic to be woven into a fabric.
Glasses are generally transparent but may be translucent or opaque.
Semi-opacity due to crystallization may be induced in many glasses by
maintaining them for a long period at a temperature just insufficient to
cause fusion. In this way is produced the crystalline, devitrified
material, known as Reaumur's porcelain. Semi-opacity and opacity are
usually produced by the addition to the glass-mixtures of materials
which will remain in suspension in the glass, such as oxide of tin,
oxide of arsenic, phosphate of lime, cryolite or a mixture of felspar
and fluorspar.
Little is known about the actual cause of colour in glass beyond the
fact that certain materials added to and melted with certain
glass-mixtures will in favourable circumstances produce effects of
colour. The colouring agents are generally metallic oxides. The same
oxide may produce different colours with different glass-mixtures, and
different oxides of the same metal may produce different colours. The
purple-blue of cobalt, the chrome green or yellow of chromium, the
dichroic canary-colour of uranium and the violet of manganese, are
constant. Ferrous oxide produces an olive green or a pale blue according
to the glass with which it is mixed. Ferric oxide gives a yellow colour,
but requires the presence of an oxidizing agent to prevent reduction to
the ferrous state. Lead gives a pale yellow colour. Silver oxide, mixed
as a paint and spread on the surface of a piece of glass and heated,
gives a permanent yellow stain. Finely divided vegetable charcoal added
to a soda-lime glass gives a yellow colour. It has been suggested that
the colour is due to sulphur, but the effect can be produced with a
glass mixture containing no sulphur, free or combined, and by increasing
the proportion of charcoal the intensity of the colour can be increased
until it reaches black opacity. Selenites and selenates give a pale pink
or pinkish yellow. Tellurium appears to give a pale pink tint. Nickel
with a potash-lead glass gives a violet colour, and a brown colour with
a soda-lime glass. Copper gives a peacock-blue which becomes green if
the proportion of the copper oxide is increased. If oxide of copper is
added to a glass mixture containing a strong reducing agent, a glass is
produced which when first taken from the crucible is colourless but on
being reheated develops a deep crimson-ruby colour. A similar glass, if
its cooling is greatly retarded, produces throughout its substance
minute crystals of metallic copper, and closely resembles the mineral
called avanturine. There is also an intermediate stage in which the
glass has a rusty red colour by reflected light, and a purple-blue
colour by transmitted light. Glass containing gold behaves in almost
precisely the same way, but the ruby glass is less crimson than copper
ruby glass. J. E. C. Maxwell Garnett, who has studied the optical
properties of these glasses, has suggested that the changes in colour
correspond with changes effected in the structure of the metals as they
pass gradually from solution in the glass to a state of crystallization.
Owing to impurities contained in the materials from which glasses are
made, accidental coloration or discoloration is often produced. For this
reason chemical agents are added to glass mixtures to remove or
neutralize accidental colour. Ferrous oxide is the usual cause of
discoloration. By converting ferrous into ferric oxide the green tint is
changed to yellow, which is less noticeable. Oxidation may be effected
by the addition to the glass mixture of a substance which gives up
oxygen at a high temperature, such as manganese dioxide or arsenic
trioxide. With the same object, red lead and saltpetre are used in the
mixture for potash-lead glass. Manganese dioxide not only acts as a
source of oxygen, but develops a pink tint in the glass, which is
complementary to and neutralizes the green colour due to ferrous oxide.
Glass is a bad conductor of heat. When boiling water is poured into a
glass vessel, the vessel frequently breaks, on account of the unequal
expansion of the inner and outer layers. If in the process of glass
manufacture a glass vessel is suddenly cooled, the constituent particles
are unable to arrange themselves and the vessel remains in a state of
extreme tension. The surface of the vessel may be hard, but the vessel
is liable to fracture on receiving a trifling shock. M. de la Bastie's
process of "toughening" glass consisted in dipping glass, raised to a
temperature slightly below the melting-point, into molten tallow. The
surface of the glass was hardened, but the inner layers remained in
unstable equilibrium. Directly the crust was pierced the whole mass was
shattered into minute fragments. In all branches of glass manufacture
the process of "annealing," i.e. cooling the manufactured objects
sufficiently slowly to allow the constituent particles to settle into a
condition of equilibrium, is of vital importance. The desired result is
obtained either by moving the manufactured goods gradually away from a
constant source of heat, or by placing them in a heated kiln and
allowing the heat gradually to die out.
[Illustration: FIG. 15.--Siemens's Continuous Tank Furnace.]
The furnaces (fig. 15) employed for melting glass are usually heated
with gas on the "Siemens," or some similar system of regenerative
heating. In the United States natural gas is used wherever it is
available. In some English works coal is still employed for direct
heating with various forms of mechanical stokers. Crude petroleum and a
thin tar, resulting from the process of enriching water-gas with
petroleum, have been used both with compressed air and with steam with
considerable success. Electrical furnaces have not as yet been employed
for ordinary glass-making on a commercial scale, but the electrical
plants which have been erected for melting and moulding quartz suggest
the possibility of electric heating being employed for the manufacture
of glass. Many forms of apparatus have been tried for ascertaining the
temperature of glass furnaces. It is usually essential that some parts
of the apparatus shall be made to acquire a temperature identical with
the temperature to be measured. Owing to the physical changes produced
in the material exposed prolonged observations of temperature are
impossible. In the Fery radiation pyrometer this difficulty is obviated,
as the instrument may be placed at a considerable distance from the
furnace. The radiation passing out from an opening in the furnace falls
upon a concave mirror in a telescope and is focused upon a
thermoelectric couple. The hotter the furnace the greater is the rise of
temperature of the couple. The electromotive force thus generated is
measured by a galvanometer, the scale of which is divided and figured so
that the temperature may be directly read. (See THERMOMETRY.)
In dealing with the manufacture of glass it is convenient to group the
various branches in the following manner:
_Manufactured Glass._
I. Optical Glass
|
II. Blown Glass
|
+----------------+-------+--------+-------------+
| | | |
A. Table glass. B. Tube. C. Sheet D. Bottles.
Special glasses and crown
for thermometers, glass.
and other special
glasses.
III. Mechanically Pressed Glass
|
+----------------+-----------------+
| |
A. Plate and rolled plate glass. B. Pressed table glass.
I. OPTICAL GLASS.--As regards both mode of production and essential
properties optical glass differs widely from all other varieties. These
differences arise primarily from the fact that glass for optical uses is
required in comparatively large and thick pieces, while for most other
purposes glass is used in the form of comparatively thin sheets; when,
therefore, as a consequence of Dollond's invention of achromatic
telescope objectives in 1757, a demand first arose for optical glass,
the industry was unable to furnish suitable material. Flint glass
particularly, which appeared quite satisfactory when viewed in small
pieces, was found to be so far from homogeneous as to be useless for
lens construction. The first step towards overcoming this vital defect
in optical glass was taken by P. L. Guinand, towards the end of the 18th
century, by introducing the process of stirring the molten glass by
means of a cylinder of fireclay. Guinand was induced to migrate from his
home in Switzerland to Bavaria, where he worked at the production of
homogeneous flint glass, first with Joseph von Utzschneider and then
with J. Fraunhofer; the latter ultimately attained considerable success
and produced telescope disks up to 28 centimetres (11 in.) diameter.
Fraunhofer further initiated the specification of refraction and
dispersion in terms of certain lines of the spectrum, and even attempted
an investigation of the effect of chemical composition on the relative
dispersion produced by glasses in different parts of the spectrum.
Guinand's process was further developed in France by Guinand's sons and
subsequently by Bontemps and E. Feil. In 1848 Bontemps was obliged to
leave France for political reasons and came to England, where he
initiated the optical glass manufacture at Chance's glass works near
Birmingham, and this firm ultimately attained a considerable reputation
in the production of optical glass, especially of large disks for
telescope objectives. Efforts at improving optical glass had, however,
not been confined to the descendants and successors of Guinand and
Fraunhofer. In 1824 the Royal Astronomical Society of London appointed a
committee on the subject, the experimental work being carried out by
Faraday. Faraday independently recognized the necessity for mechanical
agitation of the molten glass in order to ensure homogeneity, and to
facilitate his manipulations he worked with dense lead borate glasses
which are very fusible, but have proved too unstable for ordinary
optical purposes. Later Maes of Clichy (France) exhibited some "zinc
crown" glass in small plates of optical quality at the London Exhibition
of 1851; and another French glass-maker, Lamy, produced a dense thallium
glass in 1867. In 1834 W. V. Harcourt began experiments in glass-making,
in which he was subsequently joined by G. G. Stokes. Their object was to
pursue the inquiry begun by Fraunhofer as to the effect of chemical
composition on the distribution of dispersion. The specific effect of
boric acid in this respect was correctly ascertained by Stokes and
Harcourt, but they mistook the effect of titanic acid. J. Hopkinson,
working at Chance's glass works, subsequently made an attempt to produce
a titanium silicate glass, but nothing further resulted.
The next and most important forward step in the progress of optical
glass manufacture was initiated by Ernst Abbe and carried out jointly by
him and O. Schott at Jena in Germany. Aided by grants from the Prussian
government, these workers systematically investigated the effect of
introducing a large number of different chemical substances (oxides)
into vitreous fluxes. As a result a whole series of glasses of novel
composition and optical properties were produced. A certain number of
the most promising of these, from the purely optical point of view, had
unfortunately to be abandoned for practical use owing to their chemical
instability, and the problem of Fraunhofer, viz. the production of pairs
of glasses of widely differing refraction and dispersion, but having a
similar distribution of dispersion in the various regions of the
spectrum, was not in the first instance solved. On the other hand, while
in the older crown and flint glasses the relation between refraction and
dispersion had been practically fixed, dispersion and refraction
increasing regularly with the density of the glass, in some of the new
glasses introduced by Abbe and Schott this relation is altered and a
relatively low refractive index is accompanied by a relatively high
dispersion, while in others a high refractive index is associated with
low dispersive power.
The initiative of Abbe and Schott, which was greatly aided by the
resources for scientific investigation available at the Physikalische
Reichsanstalt (Imperial Physical Laboratory), led to such important
developments that similar work was undertaken in France by the firm of
Mantois, the successors of Feil, and somewhat later by Chance in
England. The manufacture of the new varieties of glass, originally known
as "Jena" glasses, is now carried out extensively and with a
considerable degree of commercial success in France, and also to a less
extent in England, but none of the other makers of optical glass has as
yet contributed to the progress of the industry to anything like the
same extent as the Jena firm.
The older optical glasses, now generally known as the "ordinary" crown
and flint glasses, are all of the nature of pure silicates, the basic
constituents being, in the case of crown glasses, lime and soda or lime
and potash, or a mixture of both, and in the case of flint glasses, lead
and either (or both) soda and potash. With the exception of the heavier
flint (lead) glasses, these can be produced so as to be free both from
noticeable colour and from such defects as bubbles, opaque inclusions or
"striae," but extreme care in the choice of all the raw materials and in
all the manipulations is required to ensure this result. Further, these
glasses, when made from properly proportioned materials, possess a very
considerable degree of chemical stability, which is amply sufficient for
most optical purposes. The newer glasses, on the other hand, contain a
much wider variety of chemical constituents, the most important being
the oxides of barium, magnesium, aluminium and zinc, used either with or
without the addition of the bases already named in reference to the
older glasses, and--among acid bodies--boric anhydride (B2O3) which
replaces the silica of the older glasses to a varying extent. It must be
admitted that, by the aid of certain of these new constituents, glasses
can be produced which, as regards purity of colour, freedom from defects
and chemical stability are equal or even superior to the best of the
"ordinary" glasses, but it is a remarkable fact that when this is the
case the optical properties of the new glass do not fall very widely
outside the limits set by the older glasses. On the other hand, the more
extreme the optical properties of these new glasses, i.e. the further
they depart from the ratio of refractive index to dispersive power found
in the older glasses, the greater the difficulty found in obtaining them
of either sufficient purity or stability to be of practical use. It is,
in fact, admitted that some of the glasses, most useful optically, the
dense barium crown glasses, which are so widely used in modern
photographic lenses, cannot be produced entirely free either from
noticeable colour or from numerous small bubbles, while the chemical
nature of these glasses is so sensitive that considerable care is
required to protect the surfaces of lenses made from them if serious
tarnishing is to be avoided. In practice, however, it is not found that
the presence either of a decidedly greenish-yellow colour or of numerous
small bubbles interferes at all seriously with the successful use of the
lenses for the majority of purposes, so that it is preferable to
sacrifice the perfection of the glass in order to secure valuable
optical properties.
It is a further striking fact, not unconnected with those just
enumerated, that the extreme range of optical properties covered even by
the relatively large number of optical glasses now available is in
reality very small. The refractive indices of all glasses at present
available lie between 1.46 and 1.90, whereas transparent minerals are
known having refractive indices lying considerably outside these limits;
at least one of these, fluorite (calcium fluoride), is actually used by
opticians in the construction of certain lenses, so that probably
progress is to be looked for in a considerable widening of the limits of
available optical materials; possibly such progress may lie in the
direction of the artificial production of large mineral crystals.
The qualities required in optical glasses have already been partly
referred to, but may now be summarized:--
1. _Transparency and Freedom from Colour._--These qualities can be
readily judged by inspection of the glass in pieces of considerable
thickness, and they may be quantitatively measured by means of the
spectro-photometer.
2. _Homogeneity._--The optical desideratum is uniformity of refractive
index and dispersive power throughout the mass of the glass. This is
probably never completely attained, variations in the sixth
significant figure of the refractive index being observed in
different parts of single large blocks of the most perfect glass.
While such minute and gradual variations are harmless for most optical
purposes, sudden variations which generally take the form of striae or
veins are fatal defects in all optical glass. In their coarsest forms
such striae are readily visible to the unaided eye, but finer ones
escape detection unless special means are taken for rendering them
visible; such special means conveniently take the form of an apparatus
for examining the glass in a beam of parallel light, when the striae
scatter the light and appear as either dark or bright lines according
to the position of the eye. Plate glass of the usual quality, which
appears to be perfectly homogeneous when looked at in the ordinary
way, is seen to be a mass of fine striae, when a considerable
thickness is examined in parallel light. Plate glass is, nevertheless,
considerably used for the cheaper forms of lenses, where the
scattering of the light and loss of definition arising from these fine
striae is not readily recognized.
Bubbles and enclosures of opaque matter, although more readily
observed, do not constitute such serious defects; their presence in a
lens, to a moderate extent, does not interfere with its performance
(see above).
3. _Hardness and Chemical Stability._--These properties contribute to
the durability of lenses, and are specially desirable in the outer
members of lens combinations which are likely to be subjected to
frequent handling or are exposed to the weather. As a general rule, to
which, however, there are important exceptions, both these qualities
are found to a greater degree, the lower the refractive index of the
glass. The chemical stability, i.e. the power of resisting the
disintegrating effects of atmospheric moisture and carbonic acid,
depends largely upon the quantity of alkalis contained in the glass
and their proportion to the lead, lime or barium present, the
stability being generally less the higher the proportion of alkali. A
high silica-content tends towards both hardness and chemical
stability, and this can be further increased by the addition of small
proportions of boric acid; in larger quantities, however, the latter
constituent produces the opposite effect.
4. _Absence of Internal Strain._--Internal strain in glass arises from
the unequal contraction of the outer and inner portions of masses of
glass during cooling. Processes of annealing, or very gradual cooling,
are intended to relieve these strains, but such processes are only
completely effective when the cooling, particularly through those
ranges of temperature where the glass is just losing the last traces
of plasticity, is extremely gradual, a rate measured in hours per
degree Centigrade being required. The existence of internal strains in
glass can be readily recognized by examination in polarized light, any
signs of double refraction indicating the existence of strain. If the
glass is very badly annealed, the lenses made from it may fly to
pieces during or after manufacture, but apart from such extreme cases
the optical effects of internal strain are not readily observed except
in large optical apparatus. Very perfectly annealed optical glass is
now, however, readily obtainable.
5. _Refraction and Dispersion._--The purely optical properties of
refraction and dispersion, although of the greatest importance, cannot
be dealt with in any detail here; for an account of the optical
properties required in glasses for various forms of lenses see the
articles LENS and ABERRATION: II. _In Optical Systems_. As typical of
the range of modern optical glasses Table I. is given, which
constituted the list of optical glasses exhibited by Messrs Chance at
the Optical Convention in London in 1905. In this table n is the
refractive index of the glass for sodium light (the D line of the
solar spectrum), while the letters C, F and G' refer to lines in the
hydrogen spectrum by which dispersion is now generally specified. The
symbol [nu] represents the inverse of the dispersive power, its value
being (n_D - 1)/(C - F). The very much longer lists of German and
French firms contain only a few types not represented in this table.